"Annoyingly, I think that the first "Mummy" film with Brendan Fraser did have an isolated score track on its first DVD release. Unfortunately, the subsequent releases have removed this (and many other special features)"

This is correct. I believe it was under the Languages or Soundtracks menu. If you let the menu play until the end, it started playing the isolated score. I have this DVD, and enjoyed listening to it from time to time.

I believe the reason this was changed, and why many DVD's don't have this anymore, is because people won't double-dip and buy the CD soundtrack.